Scaling Methods¶

This section describes scaling utility functions and methods.

Context¶

Creating well scaled models is important for increasing the efficiency and reliability of solvers. Since the standard units for IDAES are SI, oftentimes variables and constraints are considered badly scaled (values less than $$10^{-3}$$ and greater than $$10^3$$).

Scaling factors can be specified for any variable or constraint. Pyomo and many solvers support the scaling_factor suffix. IDAES, as described below, also supports the scaling_expression suffix which can be used to calculate scaling_factor values (e.g. based on the scaling factors of other variables).

To eliminate the possibility of defining conflicting scaling factors in various places in the model, the IDAES standard is to define the scaling_factor and scaling_expression suffixes in the same block as the variable or constraint that they are scaling. This ensures that each scale factor is defined in only one place, and is organized based on the model block structure.

Scaling factors in IDAES (and Pyomo) are multiplied by the variable or constraint they scale. For example, a Pressure variable in Pa units may be expected to have a magnitude of around $$10^6$$ for a specific process. To scale the variable to a more reasonable magnitude the scale factor for the variable could be defined to be $$10^{-5}$$.

Specifying Scaling¶

Suffixes are used to specify scaling factors for IDAES models.

To supply variable and constraint scaling factors, an export suffix called scaling_factor should be created in the same block as the variable or constraint. Additionally, if the scaling factor for variables or constraints will be based on other variables, a local suffix called scaling_expression should be created.

Implementing Scaling¶

While some solvers, such as Ipopt, support scaling factors, Pyomo also supplies scaling transformations for models when solver scaling is not supported.

Neither Pyomo or other solvers use the IDAES scaling_expression, so the scaling expressions must be converted to scaling factors with the calculate_scaling_factors(m, basis) function. This function replaces the variables in the scaling expression with the specified basis value, calculates the scaling factors, and puts the scaling factor in the scaling_factor suffix.

Specifically, for Ipopt, it is recommended to use the scale_constraints(m) function to scale the constraints before sending the model to the solver. To scale the variables, the user must set the nlp_scaling_method option to “user-scaling”.

Example¶

from pyomo.environ import Suffix, ConcreteModel, Var, NonNegativeReals, \
Constraint, Objective, SolverFactory
from idaes.core.util.scaling import (scale_constraints, ScalingBasis,
calculate_scaling_factors)
from math import isclose

var_value_magnitude = 1e-12
m = ConcreteModel()

m.x = Var(initialize=var_value_magnitude, domain=NonNegativeReals)
m.y = Var(initialize=var_value_magnitude, domain=NonNegativeReals)
m.z = Var(initialize=var_value_magnitude ** 2, domain=NonNegativeReals)

m.c_1 = Constraint(expr=m.x + m.y <= var_value_magnitude)
m.c_2 = Constraint(expr=m.z == (m.x * m.y))
m.obj = Objective(expr=-m.z)

# create and specify scaling factors and expressions
m.scaling_factor = Suffix(direction=Suffix.EXPORT)
m.scaling_expression = Suffix(direction=Suffix.LOCAL)
# NOTE: the direction of the scaling_expression is LOCAL
m.scaling_factor[m.x] = 1 / var_value_magnitude
m.scaling_factor[m.y] = 1 / var_value_magnitude
m.scaling_factor[m.c_1] = 1 / var_value_magnitude
m.scaling_expression[m.z] = 1 / (m.x * m.y)
m.scaling_expression[m.c_2] = 1 / (m.x * m.y)
m.scaling_expression[m.obj] = 1 / (m.x * m.y)

# calculate scaling factors from scaling expression
calculate_scaling_factors(m, basis=ScalingBasis.InverseVarScale)
# scale constraints
scale_constraints(m)
# NOTE: After the constraints are scaled, their scaling factor and expression
# are set to 1.

# call solver with user-scaling option
solver = SolverFactory('ipopt')
solver.options = {'nlp_scaling_method': 'user-scaling'}
results = solver.solve(m, tee=False)
assert (isclose(m.z.value, (0.5 * var_value_magnitude) ** 2, rel_tol=1e-3))

Scaling Expression Basis¶

The general guideline for calculating scaling factors from scaling expressions is to use the expected magnitude of the variables. The magnitude could be estimated in different ways, but the IDAES standard is the inverse variable scale. The list below shows variable scaling bases that are provided.

ScalingBasis.InverseVarScale:
Use the inverse variable scaling factors in scaling expressions.
ScalingBasis.Value:
Use the current variable values in scaling expressions.
ScalingBasis.Mid:
Use the mid-point between the upper and lower bounds in scaling expressions.
ScalingBasis.Lower:
Use the lower bound of variables in scaling expressions.
ScalingBasis.Upper:
Use the lower bound of variables in scaling expressions.
ScalingBasis.VarScale:
This is less common, but it uses the variable scales directly. This can be used if you are using alternative scaling methods with divide by the scaling factor.

Scaling Utility Functions¶

IDAES includes some utility functions to help evaluate model scaling and to auto-scale constraints.

This provides a rough check for variables with poor scaling based on their current scale factors and values. For each potentially poorly scaled variable it returns the var and its current scaled value.

Parameters: blk – pyomo block large – Magnitude that is considered to be too large small – Magnitude that is considered to be too small zero – Magnitude that is considered to be zero, variables with a value of zero are okay, and not reported. variable data object, current absolute value of scaled value

Finite difference the gradient for a constraint, objective or named expression. This is only for use in examining scaling. For faster more accurate gradients refer to pynumero.

Parameters: c – constraint to evaluate scaled – if True calculate the scaled grad (default=False) h – step size for calculating finite difference derivatives (list of gradient values, list for variables in the constraint) The order of the variables corresponds to the gradient values.
idaes.core.util.scaling.scale_single_constraint(c)[source]

This transforms a constraint with its scaling factor. If there is no scaling factor for the constraint, the constraint is not scaled and a message is logged.

Parameters: c – Pyomo constraint None
idaes.core.util.scaling.scale_constraints(blk, descend_into=True)[source]

This scales all constraints with their scaling factor suffix for a model or block. After scaling the constraints, the scaling factor and expression for each constraint is set to 1 to avoid double scaling the constraints.

Parameters: blk – Pyomo block descend_into – indicates whether to descend into the other blocks on blk. = True) ((default) – None